96 Deville computer readout error help and resetting

I do not have the car to look at it(in Fla). The car is a friends of my mothers, 47K miles she is going to sell and I'm interested in, may be able to get it cheap.
She said it has the following that come up when the car is started.
retest emission system; anti theft system default; ... may not restart ; oil change needed (was recently changed).

Does this look like a computer problem $$ or general maintenance?
Can it be reset easily (something I could tell my mother to do) or does the battery cable need to be removed? I sent her the reset procedures for the oil xhange error.

Thanks,
Glenn

Dear, Sir.
Please be advised that the very first thing to do to reset the system it is actually disconnect the battery for 24 hours...

The first error -- retest emission system; seems to be serious and in case if won't disappear after the reset might cost you big $$$.
The second -- anti theft system default; might simply appear if somebody will try to put incorrect key in the start lock ...
The third --- oil change needed (was recently changed). is erased and setled back to 100% oil life left by pressing and holding the erase/reset button when the sign will appear on the display or by pressing the information button for several times until the message will appear.
Regards.
Greg. :smokin: